The present invention relates to a polymer useful as an ingredient of a resist composition for a surface resolution process used in manufacturing semiconductor elements, etc. and to a resist composition using the polymer.
In compliance with the requirements for ultra-fine processing according to high integration of semiconductor devices, the light source of exposure used in lithography processes has recently been changed from i line beams (365 nm) to one having shorter wavelength such as KrF excimer laser (248 nm) and ArF excimer laser (193 nm) and further to F2 excimer laser (157 nm) and EB (electron beams).
As the change of the light source of exposure, the main current of the pattern formation has been shifted to one using so-called chemically amplified type resist composition wherein a polymer is chemically changed to alkali-soluble one by the act of an acid as a catalyst, which is generated from a photo-sensitive compound, followed by developing by an alkaline developing solution. In this pattern formation, because of forming the pattern with the use of an alkaline developing solution, there has been recognized such a serious problem in accordance with the tendency to scaling-down as collapse of the pattern and rounding of pattern top shape by the surface tension upon evaporation of water remaining among the resist patterns, which result in failing to give the desired shape of ultra-fine pattern.
In order to dissolve this kind of problem, there has been studied a surface resolution process comprising silylating a chemically changed surface of a resist in place of using an alkaline developing solution and then dry-developing by oxygen plasma etching technology (hereinafter abbreviated as silylated surface resolution process). However, there have been found many problems in conducting this silylated surface resolution process with the use of the so far known chemically amplified type resist compositions. Namely, for instance, when poly(p-tert-butoxycarbonyloxystyrene) disclosed in U.S. Pat. No. 4,552,833, poly(p-tert-butoxycarbonyloxystyrene/N-methylmaleimide) disclosed in EPC Publication No. 0264908, poly(p-tert-butoxystyrene/methyl methacrylate) disclosed in EPC Publication No. 0284864, poly(p-tert-butoxystyrene/fumaronitrile) disclosed in Japanese Patent Publication-Kokai- No. 223858/1991, etc. are used in this process as a polymer component, there is observed such a problem that those polymers are hardly liable to the chemical change by the acid generated by exposure to cause mere insufficient chemical change in the exposed area and insufficient silylation reaction in the next process step, which results in failing to give high contrast. On the other hand, when poly[p-(1-ethoxyethoxystyrene)] disclosed in Japanese Patent Publication-Kokai- No. 171262/1996 which is easily liable to chemical change by the acid is used in this process, there have been observed such problems that silylation is occurred even in a part of unexposed areas by the chemical change to fail to give high contrast and that no sufficient pattern formation is realized because of low heat resistance of the polymer itself. Further, in case of polymers containing many silylation liable monomer units, for example, the monomer units derived from hydroxystyrene (e.g. poly[p-(1-ethoxyethoxystyrene)/p-hydroxystyrene]) such as the resist composition disclosed in Japnese Patent Publication-Kokai- No. 273934/1994 and No. 24682/1993, there is found such a problem that silylation is occurred also in unexposed areas to fail to give the necessary contrast. Still further, in a resist composition using a polymer originally containing Si in its molecule such as poly(p-trimethylsilyloxystyrene/p-hydroxystyrene) disclosed in J. C. McFarland et al., Proc. SPIE, 920, 162 (1988), there is also found such a problem that no sufficient contrast against unexposed areas by the silylation is observed to fail to form the desired ultra-fine pattern. Particularly, in the lithography process using ArF excimer laser (193 nm) as the light source of exposure, when the polymer constructed from only the styrene type monomer units [e.g. p-(1-ethoxyethoxystyrene), p-tert-butoxycarbonyloxystyrene, styrene, etc.] is used, there is observed such a problem that the polymer have very low transmissivity against ArF excimer laser to limit the part liable to chemical change to just one small portion of the surface of a resist and to cause nonuniform chemical change in the edge of pattern having lower exposure than the center, which results in nonuniform silylation reaction. For the reasons, in case of conducting a etching process after silylation, there has been recognized such a problem that the edge of pattern is nonuniform etched to make larger the edge roughness.
Now therefore, development of a polymer capable of giving ultra-fine pattern with excellent rectangular shape by the silylated surface resolution process and development of a resist composition using the polymer have strongly been desired at the present stage.
The present invention has been accomplished taking the above mentioned situation into consideration, and its object is to provide a polymer capable of forming an ultra-fine pattern with excellent rectangular shape in a silylated surface resolution process using a chemically amplified type resist composition as a single layer or the most upper layer among multiple layers and to a resist composition using the polymer.
Namely, the present inventors have extensively studied in order to dissolve the problems mentioned above observed in a silylated surface resolution process using known resist compositions to arrive at the finding that the use of a resist composition containing the polymers shown by the general formula [1] can give high contrast between the exposed areas and unexposed areas to obtain heat resistible rectangular ultra-fine pattern, and the present invention has been accomplished on the basis of this finding.
The present invention comprises the following features in order to attain the above-mentioned object.
(1) A polymer, which is shown by the following general formula [1] 
[wherein R1 is a hydrogen atom or a methyl group, R2 and R3 are each independently a hydrogen atom or a straight chained or branched alkyl group having 1 to 6 carbon atoms (excepting a case where both of R2 and R3 are hydrogen atoms), R4 is a straight chained, branched or cyclic alkyl group having 1 to 6 carbon atoms, an aralkyl group or a phenyl group, and R2 and R3, R2 and R4 or R3 and R4 may form together a ring, respectively, R5 is a hydrogen atom, a straight chained, branched or cyclic alkyl group having 1 to 6 carbon atoms, a straight chained, branched or cyclic alkoxy group having 1 to 6 carbon atoms, a tert-butoxycarbonyloxy group, a tetrahydropyranyloxy group, an acetyloxy group or a group of xe2x80x94OCH2COOC(CH3)3, R6 is a hydrogen atom or a cyano group, R7 is a group of xe2x80x94COOR8 (in which R8 is a straight chained, branched or cyclic alkyl group having 1 to 6 carbon atoms or a bridged alicyclic hydrocarbon group having 7 to 12 carbon atoms), a cyano group or a group shown by the following general formula [8]
(wherein R9 is a hydrogen atom, a straight chained, branched or cyclic alkyl group having 1 to 6 carbon atoms or a straight chained, branched or cyclic alkoxy group having 1 to 6 carbon atoms), and R6 and R7 may form together a group of xe2x80x94COxe2x80x94Oxe2x80x94COxe2x80x94or xe2x80x94COxe2x80x94NR10xe2x80x94COxe2x80x94 (wherein R10 is a hydrogen atom, a straight chained, branched or cyclic alkyl group having 1 to 6 carbon atoms or a straight chained, branched or cyclic alkoxy group having 1 to 6 carbon atoms), k is a positive integer, l, m and n are each independently 0 or a positive integer (providing 0.1xe2x89xa6k/(k+l+m+n)xe2x89xa60.9, 0xe2x89xa61/(k+l+m+n)xe2x89xa60.9, 0xe2x89xa6m/(k+l+m+n)xe2x89xa6xe2x89xa60.8, and 0xe2x89xa6n/(k+l+m +n) less than 0.1, providing that a case where l and m are both 0 is excluded)].
(2) A polymer for a resist composition, which comprises the polymer mentioned as the above (1).
(3) A resist composition, which comprises the polymer mentioned as the above (1).
(4) A resist composition comprising a polymer shown by the following general formula [1], a compound capable of generating an acid by irradiation with actinic radiation and a solvent capable of dissolving them 
[wherein R1 is a hydrogen atom or a methyl group, R2 and R3 are each independently a hydrogen atom or a straight chained or branched alkyl group having 1 to 6 carbon atoms (excepting a case where both of R2 and R3 are hydrogen atoms), R4 is a straight chained, branched or cyclic alkyl group having 1 to 6 carbon atoms, an aralkyl group or a phenyl group, and R2 and R3, R2 and R4 or R3 and R4 may form together a ring, respectively, R5 is a hydrogen atom, a straight chained, branched or cyclic alkyl group having 1 to 6 carbon atoms, a straight chained, branched or cyclic alkoxy group having 1 to 6 carbon atoms, a tert-butoxycarbonyloxy group, a tetrahydropyranyloxy group, an acetyloxy group or a group of xe2x80x94OCH2COOC(CH3)3, R6 is a hydrogen atom or a cyano group, R7 is a group of xe2x80x94COOR8 (in which R8 is a straight chained, branched or cyclic alkyl group having 1 to 6 carbon atoms or a bridged alicyclic hydrocarbon group having 7 to 12 carbon atoms), a cyano group or a group shown by the following general formula [8]
(wherein R9 is a hydrogen atom, a straight chained, branched or cyclic alkyl group having 1 to 6 carbon atoms or a straight chained, branched or cyclic alkoxy group having 1 to 6 carbon atoms), and R6 and R7 may form together a group of xe2x80x94COxe2x80x94Oxe2x80x94COxe2x80x94 or xe2x80x94COxe2x80x94NR10xe2x80x94COxe2x80x94 (wherein R10 is a hydrogen atom, a straight chained, branched or cyclic alkyl group having 1 to 6 carbon atoms or a straight chained, branched or cyclic alkoxy group having 1 to 6 carbon atoms), k is a positive integer, l, m and n are each independently 0 or a positive integer (providing 0.1xe2x89xa6k/(k+l+m+n)xe2x89xa60.9, 0xe2x89xa61/(k+l+m+n)xe2x89xa60.9, 0xe2x89xa6m/(k+l+m+n)xe2x89xa60.8, and 0xe2x89xa6n/(k+l+m+n) less than 0.1, providing that a case where l and m are both 0 is excluded)].